Recent and upcoming legislation continues to place strict standards on emissions from internal combustion engines. As a result, various control systems have been designed and used to reduce the unwanted emissions while achieving efficiency of combustion and fuel economy. One such control system produces a swirling motion of the intake air entering the combustion chamber of the engine to aid in the mixing of the fuel and intake gases. This enhanced mixing in turn promotes optimal combustion thereby reducing emission levels in the exhaust gases. Some of these swirl control systems vary the amount of swirl energy in the intake air (thus the amount of swirl in the combustion chamber) based on the engine operating conditions. This interactive variation of swirl more effectively controls the emissions throughout the operating range of the engine. For example, in general, a high swirling motion of the air is needed in the combustion chamber at lower engine operating speeds in order to enhance the fuel/air mixing process while a lower swirling motion of the air is desirable at higher engine speeds during which the swirl energy needed to assist in the mixing process is reduced due to the increased energy derived from the incoming gases at the higher piston speeds.
Most of these variable swirl systems attempt to achieve a wide range of swirl energy levels by somehow changing the characteristics of the air flow which eventually enters the engine cylinder. For example, the variable swirl intake air control system disclosed in SAE Paper No. 871618 entitled "Improvement of Diesel Engine Performance by Variable Swirl System" includes a helically-shaped intake port formed around the intake valve on the top of an engine cylinder and a smaller "sub-port" attached to the helix or head of the helical intake port. The helical portion of the intake port imparts a high degree of angular momentum to the intake air prior to entering the cylinder thereby causing the intake air to possess a high swirl energy upon entering the cylinder. As a result, the main helical port functions to ultimately provide a large swirling motion to the intake air and therefore defines the upper limit of the desired swirl range. The sub-port concept relies on the principle of "destructive interference" of air flows in order to achieve the characteristic change necessary to achieve a wide range of swirl energy levels. The sub-port is connected to the helical port so as to direct air flowing through the sub-port into the helical portion against the flow of air in the helical port. The interaction of the air flow from the sub-port with the primary flow in the helical portion is destructive in nature and causes a net decrease in the annular momentum of the combined air flowing in the helical portion of the intake port. This reduction in the angular momentum within the helical portion of the intake port causes a drop in the swirl energy induced in the cylinder. Thus, the swirling effect is reduced as the air flow through the sub-port is increased. A similar intake port arrangement is disclosed in U.S. Pat. No. 4,466,394 wherein a swirl control valve is used to control the amount of air flowing through the sub-port. However, although effective in achieving a broad range of swirl energy levels, this type of swirl control arrangement suffers the penalty of large pressure losses which occur in a high swirl helical port thus resulting in decreased efficiency and reduced fuel economy. Although any helical intake port experiences pressure losses due to the turning of the intake air flow thru the duct, the swirl level (and therefore corresponding pressure loss) produced by a helical intake port can be controlled through proper design of the helix throat, entrance area and ramp angle. (In general, the higher the swirl level, the higher the pressure loss). Since the helical port alone must impart high swirl to the intake air at low engine speeds, the intake port of the above-discussed "sub-port" concept must be a high swirl design, thus resulting in unnecessarily large pressure losses. Moreover, the destructive nature of the air flowing from the sub-port into the helical portion during higher engine speeds creates additional undesirable pressure losses.
U.S. Pat. Nos. 4,612,903 to Urabe et al., 4,998,518 to Mitsumoto, 5,076,224 to Smith, Jr. et al. and 5,186,139 to Matsura, and SAE Paper No. 851210 all appear to disclose swirl control systems for engines which operate based on the destructive interference concept by using a control valve to vary the amount of air flowing from an auxiliary passage into a main passage. In each instance, the opening of the control valve permits more air through the auxiliary passage and results in a decrease in the swirling effect in the cylinder. Conversely, when the valve closes, the swirling effect increases. Therefore, these systems also cause increased pressure losses in the main passage due to the destructive force of the incoming air from the sub-port. Moreover, these systems all include main passages which alone create high swirling effects at low speeds/loads and, therefore, create undesirable pressure losses.
U.S. Pat. Nos. 4,481,922 to Sugiura and 5,056,486 to Johannes both disclose intake port structures which create strong swirls in the cylinder. The Sugiura patent discloses an intake port arrangement which uses a primary pipe to produce strong swirls in the combustion chamber at low loads and a secondary pipe, which joins with the primary pipe, to admit additional air at higher loads. Thus, the primary pipe is of a design which produces high swirls at low loads and therefore also creates high pressure losses. Moreover, since the secondary pipe does not direct secondary air tangentially into the air flow of the primary pipe, this design necessarily results in some degree of destructive interference and therefore pressure losses, as the secondary air collides with, and disrupts, the primary flow of air. The patent to Johannes discloses an intake port arrangement wherein a plurality of intake ports are separately connected to the cylinder head and opened sequentially as engine speed increases to create an even unidirectional swirl in the cylinder at any given engine speed. However, Johannes does not recognize the use of a helical port for creating a low swirl in the cylinder by imparting a pre-cylinder angular momentum in the intake air. Moreover, the Johannes design creates an uninterrupted continuous strong swirl turbulence throughout all engine speeds and therefore fails to recognize the importance of creating high swirl at low engine speeds while varying the swirl based on engine operating conditions.
Another example of a swirl control system is disclosed in U.S. Pat. No. 4,793,306 wherein an intake port is provided with an air nozzle positioned at an acute angle with respect to the plane of the top of the cylinder. The nozzle is operated to deliver a high speed flow of air across the intake valve opening so as to create a swirl of air in the cylinder especially during light load operation of the engine. The nozzle can also be partially or completely closed during heavy load operation to allow the main flow of air to be supplied primarily by the intake port. However, Swain relies on the nozzle to create the swirl in the cylinder and therefore does not recognize the use of a low swirl helical port to create a pre-cylinder angular momentum in the intake air which can be varied to vary the in-cylinder swirl while minimizing pressure losses in the helical port. Also, the nozzle disclosed in Swain appears to be connected to the intake passage in such a manner so as to cause the air flowing from the nozzle to collide with the intake air flowing through the intake passage causing destructive interference and undesirable pressure losses. Moreover, as a result, in order to increase the swirl using a nozzle connected apparently perpendicular to the intake passage, the Swain design requires an unnecessarily large, high speed flow air from the nozzle.
Thus, the prior art fails to disclose a swirl control system which simply and effectively controls the swirling effect in an engine cylinder throughout a wide range of engine operating conditions by varying the pre-cylinder swirl energy of the intake air while minimizing the pressure losses along the intake port.